The present invention relates to a method and apparatus for etching round templates.
In the manufacture of a round template, a metallic hollow cylinder is thinly coated on its outer surface with an organic lacquer and has a very thin wall. The metallic hollow cylinder possesses a plurality of small through-openings, which are situated very closely adjacent to one another and which are sealed or closed by the organic lacquer. To generate a pattern image, the metallic hollow cylinder, which is in the form of a metallic screen, is clamped at both ends and rotated in a device which is similar to a lathe. A laser beam is guided parallel to the longitudinal axis of the hollow cylinder and deflected by a deflecting mirror which is movable parallel to the axis, in such a manner that the beam then extends perpendicular to the surface of the hollow cylinder. The laser beam is focused by a lens arrangement in such a manner that a focal point of very small diameter lies precisely on the surface of the cylinder. When the laser beam is energized, the organic lacquer is eroded from the surface of the hollow cylinder by vaporization, in a manner corresponding to the predetermined pattern image. The laser beam is guided in the axial direction of the hollow cylinder and the hollow cylinder is rotated. In the eroded lacquer regions, which can also be designated as pattern regions, the through-openings in the hollow cylinder are accordingly free, so that viscous agents, for example a printing paste, may be pressed through them are a subsequent stage.
To generate the laser beam, a gas power laser is usually employed, which in general contains carbon dioxide as the laser medium. When such a gas laser is deenergized, depending upon the length of the deenergization time, differing energy progressions of the laser radiation are obtained. In this case, the length of the deenergization time is dependent essentially upon the laser gas composition. In the event of an incorrect choice of the deenergization time, there is a considerable risk that pattern images may arise with edge structures not sharp enough to comply with current requirements.
On the other hand, the energy of the laser radiation which is expended for the erosion of the lacquer layer is invariably chosen to be slightly greater than would correspond to the vaporization energy for the lacquer. In this case, the excess radiation energy is reflected by the metallic screen surface lying below the lacquer. This leads to a situation in which the gas laser can no longer be reliably switched.
In most cases, a carbon dioxides laser of appropriate power is excited by means of electrodynamic steady or alternating fields. In the case of a direct-current excitation, the electrodes are located in the gas stream and the emission of a laser beam is compelled by energization or deenergization of a direct voltage of several kilovolts. In the case of an alternating field excitation, the electrodes are disposed parallel to the gas stream outside the glass tubes bounding the latter and the field extends perpendicular to the tube axis through the glass tube wall, and specifically with a frequency of a few hundred kHz up to 50 MHz.
A precondition for the laser process is that the laser gas can store energy at a metastable level. However, this results in the disadvantage that in the laser gas, latent energy remains stored even after deenergization of the electrical excitation, so that residual radiation reflected from the surface of the metal cylinder can cause a discharge of the metastable energy level even after the deenergization of the electrical excitation. Non-uniform deenergization the laser beam results, so that very imprecise etch edges are obtained.
However, even during the electrical energization period of the laser, i.e. during the time interval of the electrical excitation, the aforementioned back reflections lead to undesirably high and very short pulses of the laser radiation, which in certain circumstances can cause a destruction of the surface of the metal screen. Furthermore, pulsating changes in the width of the etch line generated by the focused laser beam are obtained. Since these phenomena not only cause a loss of quality of the etching, but also involve a loss of strength of the base substrate, it is preferable to prevent them.